The invention is concerned with apparatus for, and a method of, monitoring machinery which in an abnormal operating condition exhibits a vibration frequency peculiar to or characteristic of, that condition. A typical piece of machinery of this kind and in connection with which the present invention has been developed is a liquid ring pump. In such a pump of the circular lobe type it has been found that as the compressor stalls, that is to say it operates with a malformed ring, a spike appears in the vibration spectrum at 1 and 1/3 times the compressor operating speed. This spike is either low or non-existent in a normally operating pump. It is desired to sound an alarm or close the pump down or perform some other monitoring function when this condition exists.
It will be recognized that while the invention has been developed for and has particular application in the monitoring of compressors and pumps of the liquid ring type, it is equally applicable to any machinery which exhibits the aforementioned characteristic.